1. Technical Field
The present invention relates to double-stator rotating electric machines which include a rotor, an outer stator disposed radially outside the rotor, and an inner stator disposed radially inside the rotor.
2. Description of Related Art
Japanese Patent Application Publication No. JP2007261342A discloses an in-wheel motor which includes a rotor and a pair of outer and inner stators. The rotor is connected to a wheel shaft so as to rotate together with the wheel shaft. The outer stator is fixed to a housing so as to be positioned radially outside the rotor with an outer gap formed therebetween. The inner stator is fixed to the housing so as to be positioned radially inside the rotor with an inner gap formed therebetween. That is to say, the in-wheel motor is a double-gap and double-stator motor.
Moreover, in the in-wheel motor, the outer stator includes a plurality of iron cores each having a coil wound thereon. The inner stator includes an iron core having a plurality of protruding pieces; each of the protruding pieces has a coil wound thereon. The rotor includes an annular rotor core, a plurality of outer permanent magnets and a plurality of inner permanent magnets. The rotor core is formed by laminating a plurality of thin steel sheets. The rotor core has a plurality of fitting holes that are formed in a radially outer surface of the rotor core along a circumferential direction of the rotor core. Each of the outer permanent magnets is fitted in one of the fitting holes of the rotor core. Each of the inner permanent magnets is attached on a radially inner surface of the rotor core along the circumferential direction so as to be radially aligned with one of the outer permanent magnets.
With the above configuration, magnetomotive forces of the outer and inner stators are serially arranged with each other. In other words, the in-wheel motor has a serial arrangement of the magnetomotive forces of the outer and inner stators.
Moreover, since the coils of the outer stator are provided separately from the coils of the inner stator, the parts count of the in-wheel motor and thus the number of manufacturing steps of the in-wheel motor are large. In addition, the coil end height (i.e., the axial length of coil ends that protrude from corresponding axial end faces of the cores of the outer and inner stators) is also large.
On the other hand, the magnetomotive forces of the outer and inner stators may be arranged parallel to each other. In this case, for each radially-aligned pair of the coils of the outer and inner stators, electric currents respectively flowing in the pair of the coils are opposite in phase to each other; therefore, it is possible to use a bridging wire, which radially extends across the rotor, to bridge (or electrically connect) the pair of the coils. In other words, it is possible to integrally form the pair of the coils of the outer and inner stators and the bridging wire bridging them into one piece. Consequently, with the integral formation, it is possible to reduce the coil end height.
However, with the parallel arrangement of the magnetomotive forces of the outer and inner stators, the outer magnetic flux loops (i.e., the loops of magnetic flux flowing through the outer stator and a radially outer half of the rotor) are formed separately from the inner magnetic flux loops (i.e., the loops of magnetic flux flowing through the inner stator and a radially inner half of the rotor). Moreover, both the outer magnetic flux loops and the inner magnetic flux loops flow through the rotor in the same direction. Consequently, magnetic saturation of the rotor may occur.
In comparison, in the case of serially arranging the magnetomotive forces of the outer and inner stators, it is possible to prevent magnetic saturation of the rotor from occurring. However, in this case, it may be difficult to minimize the coil end height.